Try disabling some programs which would be loaded at startup process first.....then we'll see what happens.... Hope this helps.

Examples for programs running at background:
Realplayer
Winamp
ICQ
MSN
Anti-Virus(recommend leaving it on)
Ms Office tool bar...
etc

It is also possible you've gotten a virus or work at your computer....please back up your data first if you want to reinstall the windows.